I don't know what the surface is like at the North Pole, but running on snow is very different than running on ice. And there are lots of different types of snow Loose, soft, hard, flaky, wet, dry etc...!
I love running in fresh snow, 2-3 cm deep - and hard-packed snow is quite grippy with trail shoes. Deep snow is tough, more like sand and you need to lift your knees up higher.
Ice is a different matter - though it's often easier to run than walk on ice, it's said that running is just controlled falling anyway! I've done a parkrun on a completely iced course with no issues at all.
I also have some YakTrax which work pretty well on ice and may help your confidence. Get 'em before winter as everywhere sells out as soon as the temperature drops below 0C
Compton 40, (near Reading) is an excellent one, normally around Easter time - cheap as you like, mostly off-road, not too hilly, CPs every 5 miles.
Two 20-mile laps, and there's a 20-mile option that starts at the same time.
r.e. That Cardiff Ultra, (42 mile version) I live in Cardiff, the ultrarace Cardiff 50 was my first 50-miler in 2011 and I did it again in 2013. I really don't understand why they don't keep going to Bute Park and make it 50? Weird... Having said that, I'm hearing good things about the organisers after the Gower 50, so might be worth looking at, it's a nice enough route - lots of tarmac though
That's good to hear GKD, I might look at them when it comes time to renew again.
Had heard some horror stories about Dogtag which put me off, mainly from the winter sports crowd. I guess if you're part of an organised event, carrying the correct kit and complying with the event rules things are a lot simpler.
I'm not doing The Spine but have a bit of experience in long events out in the middle of nowhere
I highly recommend a Sawyer Mini Filter, while I don't mind drinking from rivers and streams this gives you some peace of mind. About £20-25 online, pretty small and light. It'll even go in-line on a bladder.